The present invention relates to the field of oil and gas production methods and apparatus, and more particularly to the field of flowlines used in the production of oil and gas from offshore locations. More particularly still, the present invention relates to the field of break-away flowline fittings for protecting subsea Christmas trees or other equipment to which the flowlines are connected when the flowlines are subjected to an externally applied axial load and/or bending moment.
In the production of oil and gas from offshore locations, flowlines which run over or along the sea floor, such as the flowlines running from the Christmas tree of a subsea satellite well to a remote production platform or the like, are sometimes subjected to extreme externally applied axial loads or bending moments from encounters with ships' anchors, ensnarement in fishermen's nets, or encounters with other subsea hazards. It is possible for such external axial loads or bending moments to be great enough to topple or cant the wellhead and Christmas tree of such a subsea well before the flowline breaks, which of course can cause severe damage to the installation and can even result in a blowout or other catastrophic occurrence. Subsea trees are typically provided with "fail-safe" valves which would close and shut in the well if the flowline were severed. If the trees or other components are pulled over and sufficiently damaged, however, such fail-safe valves may themselves be damaged or disabled to the extent that they are unable to operate.
The danger of toppling, canting, or otherwise damaging the Christmas trees and other wellhead components before the flowlines break, when the flowlines are subjected to such an external axial load or bending moment, arises because subsea flowlines must be designed to accommodate the high hoop stresses caused by the internal fluid pressures encountered in service. From traditional stress analysis of long, thin walled tubes subjected to internal pressure, it is known that these hoop stresses are twice the axial stresses resulting from those same internal fluid pressures. Thus, in selecting the wall thickness of a pipe to be used as a flowline subjected to internal pressure, the hoop stress will determine the minimum serviceable wall thickness. The flowline wall thickness necessary to accommodate these high hoop stresses and contain the internal fluid pressures without bursting typically results in flowlines which are so strong in tension and/or bending that such flowline strength exceeds the strength or capacity of the wellhead and Christmas tree to withstand, without damage, those same external axial loads or bending moments. Thus, when dragged by a ship's anchor, for example, if the flowline breaks at all, it may well be after costly or even irreparable damage to the tree and wellhead has occurred. Even worse, as stated above it may well be after the operators have a catastrophe on their hands, such as a blowout.
Various devices have been proposed in the past to cause the axial separation of a fluid-carrying conduit at a predetermined external axial load and/or bending moment, below that required for tensile failure of the conduit material. For example, a separable and at least partially pressure balanced safety pipeline connector or joint is disclosed in U.S. Pat. No. 4,059,288, issued Nov. 22, 1977, to Harvey O. Mohr. The connector of the Mohr patent has a weak point causing it to separate at a predetermined tension load and is designed to be substantially insensitive to operating line pressure. The connector includes a housing having one axial end adapted for connection to the pipeline and an open opposite end. A pipe extension member has one end adapted for connection to the pipeline and another end adapted for telescoping, sealed insertion into the open end of the housing. A shear disk is mounted between the extension member and the housing and is designed to shear when the axial load reaches a predetermined level, permitting axial separation of the housing and extension member at loads above said predetermined level and preventing such separation at loads below said level. The housing and extension member are arranged to provide an annular chamber between them whereby fluid pressure in the chamber urges the housing and extension members axially together to balance line pressure. A port is provided through the extension member to communicate line pressure to the chamber.
U.S. Pat. No. 4,361,165, issued Nov. 30, 1982, to John F. Flory, discloses a break-away pipeline coupling with associated valves which automatically close when the pipeline separates in order to stop the flow of fluids from the pipeline. The separation of the coupling occurs as shear pins or shear studs disposed between the coupling sections fail when the coupling is subjected to an external tensile force exceeding a predetermined level. A plurality of piston and cylinder pressure compensating devices are arranged around the exterior of the coupling sections to hold them together until separation. The pressure compensating devices apply a compression load on the coupling to counteract the tension load exerted by internal pipeline pressure. The pistons move axially within cylinders in response to pressure differentials, and ports or tubing expose one side of the pistons to the internal pipeline pressure. When internal pressure produces a force which tends to separate the coupling, an opposing compressive force resulting from the pressure acting on the pistons tends to hold the coupling together. External tensile force tending to separate the coupling is resisted by the shear pins, but when a predetermined maximum external tensile force is exceeded, the shear pins fail and the coupling sections will separate.
Another example of a safety joint proposed in the past to provide a controlled weakness in a pipeline is set out in U.S. Pat. No. 4,688,827, issued Aug. 25, 1987, to Max Bassett. The Bassett joint includes a pair of tubular mandrels connectable to opposing pipe ends. A portion of the first mandrel is insertable in a part of the other mandrel, and a latch mechanism is provided between them to hold them against axial separation. A retaining shoulder on the first mandrel retains the latch in engagement and a sealing ring is disposed between the mandrels to seal them together. A fluid-tight pressure compensation chamber surrounds the portion of the first mandrel received in the second mandrel and communicates through a fluid passage with the interior of the mandrel. The seal and its associated pressure compensation chamber are sized whereby forces tending to separate the mandrels are substantially equalized by the fluid pressure in the chamber. Frangible members normally hold the two mandrels together but break when a predetermined axial load is applied, the pressure compensation chamber being axially crushable when the frangible members break, thereby allowing the limited axial movement between the mandrels. The mandrels are allowed to move axially a sufficient distance to permit the retaining shoulder to move axially to release the latch mechanism, thereby allowing the mandrels to disengage from one another.
Yet another example of a safety joint previously proposed to provide a controlled weakness in a pipeline is set out in U.S. Pat. No. 3,659,877, issued May 2, 1972, to James W. Kubasta. The Kubasta device comprises a pair of telescopingly engaged upper and lower tubular coupling members including a pair of flanges surrounding the members for holding them in fixed relationship, one flange being secured to the upper coupling member and a split flange surrounding but not secured to the lower coupling member. Frangible bolts secure the upper flange portion to the split flange with the split flange adapted to separate when excessive axial force is placed on the coupling and breaks the frangible bolts. A seal means, such as a plurality of spacers or packing rings, is disposed between the coupling members to provide a fluid tight seal between them.
While one or more of the above-described prior art devices may have met with some degree of acceptance in some circles of the offshore oil and gas industry, all of them suffer from the same primary drawback: they are all relatively complicated devices with a multiplicity of interfitting parts or components, thus necessitating relatively complex and expensive manufacturing processes. Moreover, at least the Mohr and Flory devices have internal moving parts, thereby particularly leaving them open to the possibility of failure, especially after long periods of exposure to the corrosive effects of sea water or the other usual hazards of the typical subsea environment. Moreover, all of these prior art devices include one or more seals for permitting the respective break-away devices to serve as part of a fluid-carrying conduit. This opens each of these devices to the possibility of seal failure, which again becomes more likely after the devices have remained on the sea floor for an extended period of time exposed to the harsh subsea environment, and which in turn would require potentially costly or time-consuming repair or replacement of the break-away devices. Accordingly, these prior art devices have not adequately solved the problems referred to above with respect to the need for a break-away flowline fitting to protect a subsea Christmas tree or the like from excessive externally applied axial loads and/or bending moments.
Other examples of releasable tubing or pipe string couplings designed for use in the oil and gas industry, particularly for strings of pipe used in a well bore, are shown in U.S. Pat. No. 3,727,948, issued Apr. 17, 1973, to James H. Current, and U.S. Pat. No. 3,502,353, issued Mar. 24, 1970, to Erwin Burns. These patents each disclose use of frangible means such as shear pins or shear studs to maintain upper and lower sections of pipe or tubing together, and they each require a seal between the sections to permit the couplings to carry fluid without leaking. Thus, these devices would also be unsuitable for a break-away flowline fitting for subsea use.
Frangible bolts disposed in an annular gasket-like member between upper and lower flanged structures, which bolts shear to permit the upper structure, such as a fire hydrant or light pole, to separate from or cant with respect to the lower supporting structure, are disclosed in U.S. Pat. No. 2,305,377, issued Dec. 15, 1942, to William W. Corey. Such a frangible bolt and gasket device for a fluid-carrying conduit would also require the use of seals between the flanges of the connecting structures, and again, for this reason would be unsuitable for subsea use for a break-away flowline fitting.